The present invention generally relates to a drive system for a vehicle and, more particularly, to a multi-axle vehicle where one of the drive axles is driven by an electric motor.
Conventional four-wheel drive vehicles commonly include a transfer case for selectively distributing engine power to one or both of the front and rear drive axles. Efforts to increase the efficiency, reliability, and cost effectiveness of such systems have resulted in the development of a variety of transfer cases, differentials, and torque biasing mechanisms including hybrid four-wheel-drive systems wherein the primary drive axle is mechanically driven by the engine output and the secondary or auxiliary axle is driven by an electric motor. These systems have numerous advantages including reducing the weight and packaging size of the drive system such as by eliminating the mechanical shaft between the engine and auxiliary drive wheels. The elimination of the drive shaft to the auxiliary axle also increases system modularity, simplifies assembly tasks, and commonizes the underbody configuration across vehicle platforms, all of which contribute to the overall reduction of the complexity and cost of assembly operations.
In these hybrid systems, the electric motor is typically powered by the vehicle battery. A high-power switching device, such as an inverter, receives electrical power from the battery and then transmits power to the motor. While these systems have certain desirable features, they also suffer from a variety of drawbacks including the need for a high power inverter, electronics to condition the power supplied to the inverter as well as high voltage and power requirements that reduce the system effectiveness and add undesirable weight. Further, as conventional systems rely upon a battery or engine mounted alternator/generator for the power supply, the generated power is not synchronized to the speed of the driven wheel making it difficult and costly to compensate for speed or phase differences.